JP4209765B2 - Terahertz wave imaging device - Google Patents

Description

  The present invention relates to a terahertz band coherent electromagnetic wave generation irradiation method and apparatus using a laser light source.

Using a wavelength tunable laser, it becomes possible to generate a single-frequency coherent terahertz with a variable frequency from a terahertz electromagnetic wave generating crystal such as a dielectric LiNbO ₃ or a semiconductor GaP crystal. It can be used for cancer cell detection, IC component inspection, and the like. That is, a terahertz band image of the specimen is obtained in accordance with the terahertz resonance frequency of the living body or cancer cell. For this reason, an image is obtained by locally irradiating a terahertz beam and moving the stage on which the specimen is placed.

  In a conventional method for obtaining an image, a YAG laser used as a pump light source is excited by a flash lamp, and a repetition frequency is 10 Hz to 20 Hz. Moreover, in order not to impair the S / N of the image, a high-sensitivity silicon bolometer has to be used. The silicon bolometer uses liquid helium, so it has been simplified. On the other hand, the pyroelectric detector DTGS operating at room temperature has a low S / N because it has a sensitivity of only about 1/1000 compared to a silicon bolometer. There wasn't. Since it takes several seconds per point, it takes 1 hour or more to obtain an image having 1,000 points.

  In the conventional terahertz image device, the frequency variable range is about 0.7 THz to 2.5 THz. The natural vibration bands of biomolecules related to gene abnormalities are based on intermolecular interactions and are distributed from about 0.1 THz to about 6 THz. Therefore, it is difficult to obtain an image in which many molecular species are clearly identified unless the image is obtained at a different frequency by significantly extending from the terahertz frequency range.

  The present invention eliminates such drawbacks and provides a simple method and apparatus for obtaining a clear image at a high speed and a wide variable frequency range.

  Both the pump light laser and the signal light laser are excited by a semiconductor laser to obtain a high repetition rate. These two beams are incident on a terahertz electromagnetic wave generating crystal having high output and wide frequency band characteristics. The crystal is placed on a heat dissipating mount, and even if high average power is incident, the lahertz wave generation efficiency does not decrease due to temperature rise, so that a high-speed and clear terahertz image can be obtained.

  The present invention provides a simple means for obtaining a clear, high-speed terahertz band image at a variable single frequency over a wide range of terahertz frequencies.

Best Mode for Carrying Out the Invention, Example 1

FIG. 1 shows a typical example of the configuration of the present invention. The terahertz electromagnetic wave generating crystal 1 is a GaP crystal, and the pump laser 2 and the signal light laser 3 each irradiate the GaP crystal with pulsed laser light having a repetition frequency of 300 Hz or more, typically 1 kHz. The names pump light and signal light are names given for convenience to two lasers serving as a difference frequency mixing light source. The two laser beams are incident on the GaP crystal at a predetermined angle close to parallel by the polarizing element 4. The heat generated in the GaP crystal by irradiation is radiated by the Peltier temperature control element 5 and the temperature is stabilized. The pump light laser is a YAG laser having a wavelength of 1.064 μm, and a YAG laser excited by a semiconductor laser that can be repeated at a high speed is used in order to set the repetition frequency to 1 kHz. The pumping semiconductor laser can obtain a large output and can repeat the output at a high speed .

On the other hand, the most suitable signal light laser is a Yb-doped fiber laser whose wavelength is variable in the range of 1.02 μm to 1.10 μm. In order to set the frequency difference from the pump light to 0.3 THz to 7 THz, from 1.065 μm Change to 1.091 μm or from 1.063 μm to 1.038 μm. Similar to the YAG laser, the Yb-doped fiber laser is also pumped with a semiconductor laser in order to obtain a repetition frequency of 1 kHz. A semiconductor laser pumped Nd: YLF laser may be used instead of the Yb-doped fiber laser as the wavelength tunable laser . In this case, the wavelength variable range is approximately 1.043 μm to 1.053 μm.

The beam diameters of pump light and signal light are about 1 mm. The beam diameter of the terahertz electromagnetic wave generated by the difference frequency mixing is about the same at the output end of the crystal, but in order to obtain a high resolution by making the beam nearly parallel by the parabolic mirrors 6 and 7, a tapered metal waveguide The beam diameter is narrowed by 8 to irradiate the specimen 9. The specimen 9 is, for example, a living tissue containing cancer cells that have been cut thinly, or an IC card. The specimen 9 is placed on the xy movable stage 10 and moved sequentially, and terahertz electromagnetic waves that have passed through the specimen are detected by the detector 11. The exit of the metal waveguide has a thin diameter of 0.1 mm to 0.3 mm, so that an image of the cancerous region of the specimen can be obtained with a resolution of about 300 μm.

  1 shows a configuration in which a transmission image is obtained, whereas in order to obtain a reflection image, reflected light 12 from the specimen is extracted through a beam splitter 13 and detected by a tapered metal waveguide 14 as shown in FIG. Guide to vessel 15.

  The measurement area of the specimen typically has a size of 10 mm × 10 mm, and in order to obtain an image with a resolution of 300 μm, the transmission intensity or reflection intensity of about 1,000 spots is determined for each pulse. Measure sequentially. Since the repetition frequency of the two pump lasers is 1 kHz, if one point is measured with one pulse, one transmission image can be obtained in one second. That is, it can be measured in a sufficiently fast time compared to the time for installing the specimen. In order to enable measurement of one spot per pulse, the intensity of the terahertz wave must have a sufficient S / N ratio.

  As a detector, a silicon bolometer gives a high S / N even for a weak terahertz wave, but it is not suitable for use at an inspection site because it requires liquid helium. Therefore, the pyroelectric detector DTGS operating at room temperature is used as a detector. Since DTGS has a sensitivity of 1/1000 compared to a silicon bolometer, a light source having a terahertz wave output of 50 mW at least at the maximum output frequency per pulse is required.

Further, in the case of LiNbO ₃ which is conventionally known as a variable frequency terahertz wave generator, the spectrum range is 0.7 THz to 2.5 THz. The conventional measurement alone is extremely insufficient for identifying many different substances, and it is desirable to cover a wide frequency range of one digit or more, that is, a range of 0.5 THz to 5 THz or more as a measurable range. .
Here, the measurable range is a frequency range in which an output of about 1% or more with respect to the output at the maximum output frequency can be obtained.

The terahertz electromagnetic wave intensity obtained from the GaP crystal is sufficient because the output of pump light and signal light is 50 mW to 100 mW when the output of pump light and signal light is 3 mJ per pulse. The measurable range can be obtained from a maximum of 0.3 THz to 7 THz. Therefore, in the present invention, the GaP crystal is most desirable as a terahertz electromagnetic wave generating crystal. In the case of GaP crystal, difference frequency mixing is performed by exciting phonon polaritons, and in the 1.064 μm band, due to phase matching, the two laser beams have a small crossing angle of approximately 2 degrees or less that increases with the terahertz frequency. It is necessary to make it incident .

  If a GaSe crystal having difference birefringence crystal birefringence is used, the frequency range is narrower than GaP, but two laser beams can be made parallel, so that there is an advantage that the structure of the beam incident system is simplified. A DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) crystal has a narrow variable range on the low frequency side as compared with a GaP crystal, but has an advantage that parallel incidence is possible.

  As described above, when the pulse repetition frequency is 1 kHz and the pump light and the signal light are incident at 3 mJ per pulse, the total average power incident on the GaP crystal is 6 W. When the beam diameter is 1 mm to 3 mm, the GaP crystal is locally heated and the temperature rises. As a result, the phase matching condition changes, and the difference frequency generation efficiency decreases. In order to avoid this, the GaP crystal is installed by sandwiching the crystal between the Peltier temperature-controlled heat dissipating mount 5 as shown in FIG. In order to improve the thermal contact, a soft metal 1n thin plate is sandwiched between the contact surface of the crystal and the heat dissipating mount, pressure is applied from the upper surface of the crystal, or a paste with good heat conduction is applied to improve the thermal contact.

If the pulse repetition frequency of pump light and signal light is reduced from 1 kHz to 300 Hz, the power incident on the crystal is reduced to about 2 W, and the reduction in efficiency can be reduced. The time required to obtain an image is 3 seconds, which is three times that of the first embodiment, which is a problem in an inspection by an extremely high-speed flow operation, but can maintain a practical high speed in a general inspection.
If the pump light source and the signal light source laser are mode-locked, a high-speed repetition frequency of 100 MHz or more can be obtained. However, since the output energy per pulse is small, in order to obtain one spot data with the same S / N, about 1000 pulses or more. Must be integrated.

  In order to further improve the heat dissipation from the crystal for generating terahertz electromagnetic waves and enhance the high speed performance, the cross section of the pump light and signal light incident on the crystal is made into a flat elliptical beam by the cylindrical convex lens 16 and the concave lens 16 'as shown in FIG. If the light is incident on a crystal thinned to a thickness of about 300 μm, heat conduction to the heat sink can be enhanced. If the cross-sectional areas of the beams are the same, the generation efficiency of terahertz electromagnetic waves is the same. As an example, a circular cross-section beam having a diameter of about 1 mm is formed into a thin elliptical shape of 300 μm × 3 mm using an elliptical lens, the difference frequency generating crystal is made 300 μm thick, and is sandwiched between heat dissipation mounts. Since the generated terahertz electromagnetic wave has the same flat shape at the output end, the circular cross section is returned to the circular cross section by the cylindrical lens convex lens 17 and the concave lens 17 ′ made of a material having high terahertz wave transmittance such as polyethylene. As in FIG. 1, the light enters the metal waveguide.

  It is a figure which shows the structure of Example 1 in the case of measuring transmission intensity.   It is a figure which shows the structure of Example 1 in the case of measuring reflection intensity.   6 is a diagram illustrating a configuration of Example 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Crystal for terahertz electromagnetic wave generation 2 ... Pump light laser 3 ... Signal light laser 4 ... Polarizer 5 ... Radiation mount 6, 7 ... Non-axial parabolic mirror 8 ... Metal waveguide, metal waveguide taper part 9 ... Specimen 10 ... Stage 11 ... Detector 12 ... Reflected terahertz wave 13 ... Terahertz beam splitter 14 ... Metal waveguide 15 ... Detectors 16, 16 '... Convex convex lens and concave lenses 17, 17' for near infrared rays ... Convex lens for terahertz electromagnetic wave And concave lens

Claims (3)

First and second single frequency pulse lasers pumped by a pulsed semiconductor laser having a repetition frequency of 300 Hz or more, a terahertz wave generating GaP crystal having a difference frequency between the two single frequency pulse lasers , and the GaP A heat dissipating mount for efficiently dissipating the crystal and controlling the temperature ; and means for allowing two beams from the single frequency pulse laser to enter the GaP crystal with a predetermined phase matching angle with each other , and the two single frequency pulse lasers and a desired single frequency coherent light in the range of 7THz from 0.3 THz, the terahertz wave generator for generating a terahertz wave having a repetition frequency of more than 300Hz by at least one is a variable frequency,
A sample moving stage on which a sample is placed ;
A metal waveguide for introducing the terahertz wave into the specimen ;
Terahertz wave detector for detecting the terahertz wave transmitted through the specimen
A terahertz wave imaging device for obtaining a transmission image in which the terahertz wave is transmitted through the specimen. First and second single frequency pulse lasers pumped by a pulsed semiconductor laser having a repetition frequency of 300 Hz or more, a terahertz wave generating GaP crystal having a difference frequency between the two single frequency pulse lasers , and the GaP A heat dissipating mount for efficiently dissipating the crystal and controlling the temperature ; and means for allowing two beams from the single frequency pulse laser to enter the GaP crystal with a predetermined phase matching angle with each other , and the two single frequency pulse lasers and a desired single frequency coherent light in the range of 7THz from 0.3 THz, the terahertz wave generator for generating a terahertz wave having a repetition frequency of more than 300Hz by at least one is a variable frequency,
A sample moving stage on which a sample is placed ;
A terahertz wave detector for detecting the terahertz wave reflected from the specimen ;
Metal waveguide for introducing the terahertz wave reflected from the specimen into the terahertz wave detector
A terahertz wave imaging device for obtaining a reflected image in which the terahertz wave is reflected from the specimen. The beam of the two single-frequency pulsed laser, in the GaP crystal, the terahertz according to claim 1 or 2 the cylindrical lens and a cylindrical concave lens such that the elliptical cross-section, respectively, characterized in that it is located Wave imaging device.

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